1. Field of the Invention
This invention relates generally to a brushless direct current (BLDC) motor assembly, and more particularly toward an improved method and structure for attaching permanent magnet segments on a rotor of the BLDC motor assembly.
2. Related Art
With the introduction of electronic control systems for electric motors, the industry objectives of long life, efficiency, reliability and low EM interference have become achievable. This is, in part, due to the advent of brushless direct current (BLDC) motor technology.
BLDC motor assemblies include a rotor assembly which is disposed for powered rotation within a stator in response to an electromagnetic field generated by the stator. The rotor of a BLDC motor includes a plurality of magnetic segments arrayed in equal arcuate increments about its exterior surface. Typical examples include 4 magnetic segments, each spanning approximately 90° of the rotor circumference. Such magnet segments may be of the so-called neo-magnet type.
Typically, the magnet segments are coated with a thin layer of protective material and then affixed to an underlying hub section of the rotor using a bonding adhesive. In circumstances where the coating material is not securely adhered to the magnet segment, delamination can occur during operation. This catastrophic failure of magnet separation from the underlying hub surface can lead to motor damage.
The prior art has suggested techniques other than adhesive for retaining magnet segments to the rotor in a BLDC motor assembly. For example, U.S. Pat. No. 5,563,636 to Stark, issued Oct. 8, 1996, discloses a rotor assembly wherein magnet segments are secured in their operative position upon the rotor using a sleeve-like shell. However, a shell of this type adds undesirable weight, expense and complexity to the motor assembly.
Other examples of prior art attempts to securely attach magnet segments to a permanent magnet type rotor include U.S. Pat. No. 4,625,135 to Kasabian, issued Nov. 25, 1986. In this example, the permanent magnet segments are affixed with threaded fasteners. Again, a technique such as this contributes substantially to the manufacturing assembly time, and provides additional failure modes for the motor assembly, as well as adding substantially to the component complexities.
Accordingly, there is a need for an improved method and design for attaching permanent magnet segments into an operative array on a rotor for a brushless direct current electric motor assembly which is strong, light weight, and efficiently accomplished in high-volume production settings.